Pleiotropy and Non-Nuclear Inheritance

A single gene codes for a single protein, but that protein can get used all over the body. If the gene's product sits in a pathway that feeds into skin, bone, eyes, and heart tissue, a mutation in that one gene breaks all four at once. That is pleiotropy: one genetic change, many downstream effects that look unrelated until you trace them back to the shared protein. Marfan syndrome works this way. One mutated gene, fibrillin-1, weakens connective tissue everywhere it is used, so a single patient gets long limbs, lens dislocation, and a weak aorta from one broken gene.

Non-nuclear inheritance breaks Mendel's rules in a different way: by location, not by connection. Mitochondria and chloroplasts carry their own small circular genomes, separate from the nucleus. Sperm contribute almost no cytoplasm to the zygote, so mitochondria (and in plants, chloroplasts) come almost entirely from the egg. That makes inheritance strictly maternal: a mother passes her mitochondrial DNA to every child, sons and daughters alike, but only her daughters can pass it on again. A pedigree for a mitochondrial disease shows every child of an affected mother affected, generation after generation down the maternal line, while affected fathers have zero affected children.

Both ideas sit next to Mendelian crosses in this unit as exceptions worth knowing cold. Pleiotropy breaks the assumption that one gene equals one trait. Maternal inheritance breaks the assumption that alleles segregate 50-50 from each parent. Neither produces the clean 3:1 or 9:3:3:1 ratios a chi-square test expects from nuclear genes, so recognizing them early saves you from forcing the wrong model onto the data.